There is provided a process for oligomerizing propylene by contacting propylene with silica modified zeolites. The modification of the zeolite with silica results in a product which is enriched in hydrocarbons falling within the boiling range of jet fuel and diesel fuel.
Propylene oligomerization over acidic catalysts is a known reaction. Over HZSM-5 in a fixed bed, propylene oligomerizes to a nearly Gaussian distribution of products from C.sub.6 to C.sub.36. These products correspond to gasoline, jet fuel, diesel fuel, and a heavy fraction. For the purposes of the present disclosure these fractions are defined in terms of the following boiling ranges:
gasoline=less than 330.degree. F. (166.degree. C.) PA1 jet fuel=330.degree.-480.degree. F. (166.degree.-249.degree. C.) PA1 diesel fuel=480.degree.-650.degree. F. (249.degree.-343.degree. C.) PA1 heavy fraction=greater than 650.degree. F. (343.degree. C.)
The heavy fraction is also referred to herein as the 650+.degree. F. fraction or the 650+ F. range product.
Silica modified ZSM-5 can be prepared by impregnation of a silicon containing compound followed by air calcination. U.S. Pat. No. 4,477,583 describes methods for impregnating the exterior surface of zeolites with amorphous silica, whereas U.S. Pat. Nos. 4,145,315; 4,100,219 and 4,060,568 describe methods for impregnating the interior pore space of zeolites with amorphous silica. In accordance with the present invention, it has now been observed that propylene oligomerization over silica modified HZSM-5, modified by this latter technique, gives a product distribution different from that of the parent HZSM-5. The 650+.degree. F. fraction is selectively reduced with a corresponding increase in the lighter fractions. Recycle of the gasoline fraction can give ultimate yields of jet and diesel fuels significantly higher than the corresponding yields using unmodified HZSM-5.